The process of files being damaged resulting from some hardware or software failure is called data corruption and this is one of the main problems that Internet hosting companies face since the larger a hard disk is and the more info is stored on it, the much more likely it is for data to get corrupted. There're various fail-safes, but often the data becomes damaged silently, so neither the particular file system, nor the administrators detect a thing. As a result, a damaged file will be treated as a good one and if the HDD is a part of a RAID, that file will be copied on all other drives. In theory, this is for redundancy, but in practice the damage will be worse. Once a file gets corrupted, it will be partly or entirely unreadable, therefore a text file will not be readable, an image file will show a random combination of colors if it opens at all and an archive will be impossible to unpack, so you risk sacrificing your site content. Although the most commonly used server file systems include various checks, they quite often fail to discover some problem early enough or require a long period of time to be able to check all the files and the web server will not be functional for the time being.